1. Field of the Invention
The present invention relates to a high voltage power supply circuit and, more particularly, relates to a high voltage power supply circuit for supplying a high voltage to the anode of a cathode ray tube (CRT) used as a display of a television receiver and a computer display.
2. Description of the Related Art
A CRT is generally used as a display of a television receiver and a computer display (hereinafter referred to as a monitor). The CRT is arranged to apply a high positive voltage to an anode thereof, so that electron emitted from a cathode thereof is attracted to and impinged at a high speed on the phosphor coated on a phosphor screen. A circuit for supplying such a high voltage to the anode is referred to as a high voltage power supply circuit.
FIG. 9 is a block diagram showing an arrangement of a well-known high voltage power supply circuit, in which FIG. 9A shows the fundamental arrangement of the high voltage power supply circuit, and FIG. 9B shows an example of the arrangement of the high voltage power supply circuit formed by taking the rising time of a high output voltage into consideration. FIG. 10 is an explanatory diagram showing an equivalent circuit of a load side of the high voltage power supply circuit.
As shown in FIG. 9A, the high voltage power supply circuit for driving a CRT 101 of a monitor with a high voltage includes a high voltage stabilizing circuit 102 for stabilizing the high output voltage supplied to the CRT 101, a resonance-type power supply circuit 103 for generating a flyback pulse in synchronous with a horizontal drive signal, a flyback transformer 104 for boosting the flyback pulse to a desired voltage value, and a high voltage detection circuit 105 for detecting the change of the boosted high output voltage.
According to such an arrangement, the high voltage generated in the flyback transformer 104 is detected by the high voltage detection circuit 105 and the detected value of the high voltage is fed back to the high voltage stabilizing circuit 102. In accordance with the detected high output voltage, the high voltage stabilizing circuit 102 varies a peak value of the flyback pulse which is generated by the resonance-type power supply circuit 103 in synchronous with the horizontal drive signal, thereby stabilizing the high output voltage.
In order to improve the responsibility of the flyback pulse at the time of the abrupt change of the load of the CRT, as the method for varying the peak value of the flyback pulse, the method for varying the on period of a main switching element of the resonance-type power supply circuit 103 in accordance with the change of the high output voltage in synchronous with the horizontal drive signal has been employed, in place of the method for varying the driving voltage of the resonance-type power supply circuit 103.
However, as shown in FIG. 10, the load of the CRT is represented by a capacitive equivalent circuit in which a capacitor 106 and a variable resistor 107 are connected in parallel. Thus, when the power supply switch of the monitor is turned on, the rising time of the high output voltage supplied to the anode of the CRT 101 becomes long, and hence it takes a long time to display an image on the screen of the CRT.
In order to obviate such a problem, conventionally, a capacitor 108 or a series circuit of the capacitor 108 and a diode 109 is connected between the ground and a node on a detection line from the high voltage detection circuit 105 to the high voltage stabilizing circuit, as shown in FIG. 9B. According to such an arrangement, the detected voltage detected by the high voltage detection circuit 105 is lowered during a period determined by the capacitance of the capacitor after the turning-on of the power supply switch of the monitor. As a result, during the certain period after the turning-on of the power supply switch of the monitor, the high voltage stabilizing circuit 102 forcedly increases the peak value of the flyback pulse of the resonance-type power supply circuit 103, so that the voltage higher than the original voltage is supplied to the anode of the CRT 101, thereby shortening the rising time of the high output voltage.
However, when the capacitor is connected to the node on the detection line for feeding back the high detection voltage to the high voltage stabilizing circuit 102 in this manner, there arises such a problem that the high-frequency gain of the detection line is reduced, and so the responsibility as to the abrupt change of the load of the CRT is degraded in the normal operation period upon the lapse of a certain period after the turning-on of the power supply switch of the monitor.